System and method for underwater distance measurement

ABSTRACT

At least some of the exemplary embodiments are a method. The method includes obtaining, at an underwater imaging device, a stream of images of geophysical surveying equipment, wherein the geophysical surveying equipment includes a target pattern having a calibrated image size. The geophysical surveying equipment is tracked using the image stream and the target pattern. The method further includes capturing from the image stream, a single image of the images of the geophysical surveying equipment, wherein the single image comprises an image of the target pattern having an apparent size. Based on the calibrated image size and the apparent size, a distance between the underwater imaging device and the geophysical surveying equipment is determined.

CROSS-REFERENCE TO RELATED APPLICATIONS

This disclosure claims the benefit of U.S. Provisional Application Ser.No. 61/897,544 filed Oct. 30, 2013 titled “Marine Streamer InertialNavigating Drag Body”, and U.S. Utility application Ser. No. 14/183,615filed Feb. 19, 2014 titled “Marine Streamer Inertial Navigating DragBody” of which this application is a continuation-in-part, and whichapplications are incorporated by reference herein as if reproduced infull below.

BACKGROUND

Geophysical surveying (e.g., seismic, electromagnetic) is a techniquewhere two- or three-dimensional “pictures” of the state of anunderground formation are taken. Geophysical surveying takes place notonly on land, but also in marine environments (e.g., oceans, largelakes). Marine geophysical surveying systems frequently use a pluralityof sensor streamers (long cables), which contain one or more sensors todetect energy emitted by one or more sources and subjected tointeraction with underground formations below the water bottom.Deployment of sensor streamers and sources often utilizes additionalequipment, such as paravanes, lead cables, lateral or depth controldevices, and buoys to properly align and maintain the sensor streamersand sources.

Sensor streamers such as those employed in marine geophysical surveyingmay be more than 10 kilometers in length. A plurality of such sensorstreamers that are spaced apart may be towed in a body of water behind avessel. During the operation, it may be advantageous to know andmaintain the position of the sensor streamers in the body of water toimprove data quality. Entanglement of the streamers may be avoidablewhen the position of sensor streamers in the body of water is known andmaintained.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of exemplary embodiments, reference will nowbe made to the accompanying drawings in which:

FIG. 1 shows an overhead view of a marine survey system in accordancewith at least some embodiments;

FIG. 2 shows a side elevation view of a sensor streamer illustrating theuse of tail buoys in the related art;

FIG. 3 shows a side elevation view of a sensor streamer in accordancewith at least some embodiments;

FIG. 4A is a side elevation view of a portion of a sensor streamer infurther detail in accordance with at least some embodiments;

FIG. 4B is a cross section through the side elevation view of FIG. 4A;

FIG. 5 is a block diagram of inertial navigation components which may beused in a drag body in accordance with at least some embodiments;

FIG. 6 is a block diagram of an inertial navigation system which may beused in a drag body in accordance with at least some embodiments;

FIG. 7A is a flow chart of a method in accordance with at least someembodiments;

FIG. 7B is a flow chart, in further detail, of a portion of the methodin FIG. 7A;

FIG. 8 is a block diagram of a measurement system in accordance with atleast some embodiments;

FIG. 9A is a side elevation view of a portion of a sensor streamer inaccordance with at least some embodiments;

FIG. 9B is a cross section through the side elevation view of FIG. 9A;

FIG. 10 is a flow chart of a method in accordance with at least someembodiments;

FIG. 10A schematically illustrates a portion of the method of FIG. 10;and

FIG. 11 is a side elevation view of a source unit in accordance with atleast some embodiments.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, different companies may refer to a component by differentnames. This document does not intend to distinguish between componentsthat differ in name but not function. In the following discussion and inthe claims, the terms “including” and “comprising” are used in anopen-ended fashion, and thus should be interpreted to mean “including,but not limited to . . . .” Also, the term “couple” or “couples” isintended to mean either an indirect or direct connection. Thus, if afirst device couples to a second device, that connection may be througha direct connection or through an indirect connection via other devicesand connections.

“Cable” shall mean a flexible, load carrying member that also compriseselectrical conductors and/or optical conductors for carrying electricalpower and/or signals between components.

“Rope” shall mean a flexible, axial load carrying member that does notinclude electrical and/or optical conductors. Such a rope may be madefrom fiber, steel, other high strength material, chain, or combinationsof such materials.

“Line” shall mean either a rope or a cable.

“Survey equipment” shall mean equipment deployed or towed in a body ofwater during a geophysical survey (e.g., seismic, electromagnetic) ofunderground formations.

“Operationally deployed”, in reference to a sensor streamer or drag bodycoupled to a sensor streamer, shall mean that the sensor streamer is atan operational depth for a particular marine geophysical survey.

“Leading end” of a device shall mean a leading portion of the devicewhen the device is towed through water during normal operations, butshall not be read to require the device to be towed. That is, a leadingend shall still be considered the leading end when the device isstationary.

“Global positioning system” or “GPS” shall mean a system which receivessignals from a constellation of satellites orbiting the earth, and whichdetermines a location based on receipt of the signals from thesatellites.

“Operational buoyancy” shall refer to the buoyancy of a drag body whilecoupled to a sensor streamer in normal operation. The presence or use ofan emergency buoyancy system that activates to prevent loss the dragbody (e.g., to prevent loss if the sensor streamer is severed) shall notbe considered to affect “operational buoyancy.”

“Constant”, in reference to operational buoyancy of a drag body, shallmean that the buoyancy of the drag body is not changed by virtue ofoperation of controllable means associated with the drag body, such asballast weights, ballast chambers, and the like. Buoyancy changes of adrag body caused by changes in depth, water temperature, and/or watersalinity shall not obviate the status of operational buoyancy as“constant.” Moreover, “constant” is not intended to imply absoluteconstancy but is intended to mean unchanged within the normal range ofvariability of operational parameters.

“Inertial measurement unit” shall mean an electronic device thatmeasures at least acceleration and orientation. An inertial measurementunit may be combined with a computer system to create an inertialnavigation system to calculate position by way of dead reckoning.

“Relative positioning unit” shall mean a system that provides anindication of distance to or from an object. The fact that an absoluteposition may be calculable using the indication of distance if theabsolute position of the object is known shall not obviate the status ofa system as a “relative positioning unit.”

“Fully submerged” shall mean a device is at, as measured from a point onthe device nearest the surface, one-fourth of a vertical height of thedevice beneath the surface of a body of water as the surface is definedin the absence of wave motion. As used herein, a device shall be fullysubmerged in the presence of wave motion if excursions of the device inthe water transiently reduce its depth to less than the foregoingamount.

“Correct” and “correcting”, in relation to a calculated position of aninertial navigation system, shall mean reducing positional drift usingdata derived from a source other than an inertial measurement unit.Determining a new or next calculated position using data from aninertial measurement unit shall not be considered to taking action to“correct” or “correcting” of positional drift.

“Exemplary,” as used herein, means serving as an example, instance, orillustration.” An embodiment described herein as “exemplary” is notnecessarily to be construed as preferred or advantageous over otherembodiments.

“Target object”, in the context of a distance determination betweenin-sea equipment and an underwater imaging device, shall mean an in-seaequipment the distance to which is to be determined by another in-seaequipment including the underwater imaging device. A “target object”,for succinctness of terminology, may be also be referred to as a“target”, and “target object” and “target” shall be synonymous.

“Target pattern”, in the context of a target object shall mean apattern, design, figure or the like affixed or attached to the targetobject.

As used herein, the singular forms “a”, “an”, and “the” include singularand plural referents unless the content clearly dictates otherwise.Furthermore, the word “may” is used throughout this application in apermissive sense (i.e., having the potential to, being able to), not ina mandatory sense (i.e., must).

DETAILED DESCRIPTION

The following discussion is directed to various embodiments of theinvention. Although one or more of these embodiments may be preferred,the embodiments disclosed should not be interpreted, or otherwise used,as limiting the scope of the disclosure or the claims. In addition, oneskilled in the art will understand that the following description hasbroad application, and the discussion of any embodiment is meant only tobe exemplary of that embodiment, and not intended to intimate that thescope of the disclosure or the claims, is limited to that embodiment.

Various embodiments are directed to a drag body to be used inconjunction with a marine geophysical surveying system. Moreparticularly, various embodiments are directed to drag body devicescoupled to the ends furthest from the survey vessel (i.e. the distalends) of respective sensor streamers, where the drag body devicesinclude an inertial navigation system used to determine position of thedrag body, and thus position of the distal end of the sensor streamer.More particularly still, various embodiments are directed to a drag bodywith an internally disposed inertial navigation system such thatposition of the drag body, and thus the position of the distal end ofthe sensor streamer, can be determined in spite of the drag bodyremaining submerged for extended periods of time. The specificationfirst turns to a description of an example marine surveying system.

FIG. 1 shows an overhead view of a marine survey system 100 inaccordance with at least some embodiments. In particular, FIG. 1 shows asurvey vessel 102 having onboard equipment 104, such as navigation,energy source control, and data recording equipment. Survey vessel 102is configured to tow one or more sensor streamers 106A-F through thewater. While FIG. 1 illustratively shows six sensor streamers 106, anynumber of sensor streamers 106 may be used. In at least someembodiments, for example in a two-dimensional (2D) survey, a singlestreamer may be used.

The sensor streamers 106 each comprise a plurality of sensors 116. Thetype of sensors 116 associated with each sensor streamer 106 depends onthe survey type. For example, for marine seismic surveys, each sensor116 may be a hydrophone, a geophone, or a hydrophone and geophone pair.In another example of sensors 116, for marine electromagnetic surveys,each sensor 116 may be an electric field detector, a magnetic fielddetector, or a combination electric and magnetic field detector.Likewise, a combination of seismic and electromagnetic sensors may beutilized on one or more of the sensor streamers 106. The type of sensoris independent of the design and use of drag bodies in accordance withthe example embodiments. As would be understood by one of ordinary skillin the art with the benefit of this disclosure, geophysical surveys thatutilize deep-towed streamers may especially benefit from drag bodies asherein disclosed.

As described above, in a seismic survey, such sensors may detect seismicsignals which may be generated by source units 131 and reflected by thesea floor and the geologic formations lying beneath. Source units 131may be coupled to survey vessel 102 via umbilical cables 133. Umbilicalcables 133 may include a strength member to couple the towing load tosurvey vessel 102, and may convey electrical power electrical andoptical communications between source units 131 and survey vessel 102.Source units 131 may comprise a plurality of seismic sources, forexample, marine vibrators or air guns. In at least some embodiments,source units may be towed by another vessel. Source units 131 will befurther described in conjunction with FIG. 11 below. In FIG. 1, twosource units are shown for ease of illustration. In at least someembodiments of marine survey system 100, other numbers of source units131 may be used, for example six or more source units may be used.

The sensor streamers 106 are coupled to survey equipment that maintainsthe sensor streamers 106 at selected lateral positions with respect toeach other and with respect to the survey vessel 102. For example, thesurvey equipment may comprise two paravane tow lines 108A and 108N eachcoupled to the vessel 102 by way of winches 110A and 110B, respectively.The winches enable changing the deployed length of each paravane towline 108. The second end of paravane tow line 108A may be coupled to aparavane 112, and the second end of paravane tow line 108B may becoupled to paravane 114. In each case, the tow lines 108A and 108B maycouple to their respective paravanes through respective sets of linescalled a “bridle”. The paravanes 112 and 114 are typically configured toprovide a lateral force component to the various elements of the surveysystem when the paravanes are towed in the water. The combined lateralforces of the paravanes 112 and 114 separate the paravanes from eachother until the paravanes put one or more spreader lines 120, coupledbetween the paravanes 112 and 114, into tension. The paravanes 112 and114 either may couple directly to the spreader line 120 or asillustrated may couple to the spreader line by way of spur lines 122Aand 122B.

The sensor streamers 106 each may be each coupled, at the ends nearestthe survey vessel 102 (i.e., the proximal ends) to a respective lead-incable termination 124A-F. The lead-in cable terminations 124 may becoupled to or associated with the spreader lines 120 so as to bettercontrol the lateral positions of the sensor streamers 106 with respectto each other and with respect to the survey vessel 102. Electricaland/or optical connections between the appropriate components in theonboard equipment 104 and the sensors (e.g., 116 in the streamers 106may be made using lead-in cables 126A-F. Likewise, lead-in cables 126A-Fmay also provide electrical and/or optical connections to the devicescoupled to or within the sensor streamers (such as the acousticpositioning units 142A-F discussed more below, and the drag body devices160A-F also discussed more below). Much like the tow lines 108associated with respective winches 110, each of the lead-in cables 126may be deployed by a respective winch or similar spooling device suchthat the operationally deployed length of each lead-in cable 126 can bechanged.

Each sensor streamer 106 may include acoustic positioning units 142. Inparticular, each sensor streamer 106A-F may include a respective arrayof acoustic positioning units 142A-F, disposed at locations along alength of the sensor streamer. Acoustic positioning units 142 maycomprise an acoustic transducer for communicating via acoustic wavestransmitted through the water. Communications may be between sensorstreamers, between a sensor streamer and the survey vessel, and asdescribed further below, between the drag bodies and sensor streamers,and such communications may be used to provide data indicative of therelative positions of the acoustic positioning units. For example, adrag body communicating with an acoustic positioning unit at aparticular location on a sensor streamer may determine data indicativeof a relative position of the drag body and the location of the acousticpositioning unit on the sensor streamer. In at least some otherembodiments, optical positioning units may be used to provide therelative positioning function provided by the acoustic positioning unitsin the illustrative embodiment. Such optical positioning units mayemploy electromagnetic radiation at optical wavelengths suitable fortransmission through sea water rather than acoustic signals, but thecommunications principles with respect to the exchange of data areotherwise the same.

Still referring to FIG. 1, in many cases the sensor streamers 106 willbe associated with a plurality of streamer positioning devices. Forexample, the sensor streamers 106A-F may be associated with streamerpositioning devices 150A-F, respectively, shown coupled on the proximalend of the sensor streamers. In many cases, the streamer positioningdevices 150A-F may provide only depth control, as the lateral spacing ofthe sensor streamers near the proximal ends may be adequately controlledby the spreader cable 120, and twisting (i.e., rotation about the longaxis of the sensor streamer) may not be an issue close to the lead-incable terminations 124A-F. Further, the sensor streamers 106A-F may beassociated with streamer positioning devices 152A-F, respectively, showncoupled further from the proximal ends of the sensor streamers 106A-F.The streamer positioning devices 152A-F may provide not only depthcontrol, but also lateral positional control and may assist inpreventing twisting experienced by the sensor streamers. In some caseseach sensor streamer 106 may be 1000 to 10000 meters in length, and maycomprise 20 or more streamer positioning devices.

In example embodiments, one or more sensor streamer 106A-F may beassociated with a respective drag body 160A-F. Each drag body 160A-F maymechanically couple to the distal end of its respective sensor streamer106A-F, and may provide mechanical drag in the water to aid in keepingthe sensor streamers in proper physical orientation. Moreover, in thevarious embodiments each drag body may include an inertial navigationsystem (discussed in greater detail below) such that the each drag bodycan determine its position. In some embodiments each drag body 160 maybe communicatively coupled to the onboard equipment 104 of the surveyvessel 102, and thus may be indirectly communicatively coupled to otherdrag bodies 160. Before delving into the specifics of the inertialnavigation system associated with each drag body, the specificationfirst turns to a discussion of a related-art system in order to drawdistinctions between use of drag bodies and use of tail buoys.

FIG. 2 shows a side elevation view of a sensor streamer 200 to furtherdescribe use of tail buoys in the related-art. While only one sensorstreamer 200 is visible in FIG. 2, it will be understood that many suchsensor streamers 200 may be present at similar depths but at differenthorizontal positions. In particular, FIG. 2 shows a sensor streamer 200being towed in a direction indicated by arrow 202 by survey vessel 204.In some embodiments, the forward portion of the sensor streamer may beassociated with a lead buoy 206, where lead buoy 206 may help maintainthe depth of the sensor streamer 200 and/or associated portion of thespreader line; however, in other cases the lead buoy 206 may be omitted,or other buoys (e.g., buoys associated with the spreader line (not shownin FIG. 2)) may perform similar functions. FIG. 2 also illustratesrelated-art use of a tail buoy 208. Tail buoy 208 may couple to thedistal end of the sensor streamer 200 by any suitable mechanism, such asline 210, sometimes referred as a “dead section”. In some cases, tailbuoy 208 may ride along the surface of the water during the marinegeophysical survey and at least partially support the sensor streamer200 at the selected depth D below the surface of the water. Between theproximal end of the sensor streamer 200 and the distal end of the sensorstreamer 200, the streamer positioning devices (not specifically shown)may help with localized depth control of the sensor streamer.

Sensor streamers have a tendency to cross and tangle, which may causesensor streamers having tail buoys (such as tail buoy 208 of FIG. 2) totangle with other tail buoys. Entanglement of tail buoys may also occurduring deployment or retrieval of sensor streamers 200. Furthermore, thetail buoy 208, and particularly the line 210, may entangle withobstacles, such as debris, fishing gear, offshore structures, and icefloes. Tail buoys that are entangled may require manual untangling,which is time consuming and costly. Unless a nearby repair vessel hasthe capability and availability to untangle the tail buoys, the surveyoperation may be suspended so that the survey crew may manually untangletail buoys. Moreover, tail buoys may impart unwanted motion and/ormechanical noise in rough seas, which mechanical noise may translateinto unwanted noise in the recorded signals in seismic surveys.

In other related-art systems, tail buoys may be configured to becontrollable. For example, a tail buoy may be configured to be submergedon command from the survey vessel. However, in the related-art systemswhere tail buoys submerge on command the tail buoys may periodicallysurface in order for telemetry hardware in the tail buoy to acquireposition data. For example, where the telemetry hardware of a tail buoyincludes Global Positioning System devices, the tail buoy mayperiodically surface in order to receive satellites signals to determineposition. The requirement of periodically surfacing to establishposition may require the geophysical survey to cease, thus increasingthe time and cost. Moreover, in related-art systems the tail buoystypically utilize a complex design with many moving parts in order toselectively submerge and surface. In some cases, such as marinegeophysical surveys using deep tow sensor streamers (e.g.,electromagnetic surveys) the depth of the sensor streamer may hinder useof tail buoys.

FIG. 3 shows a side elevation view of a sensor streamer 106 utilizingdrag body 160 in accordance with example embodiments including some deeptow example embodiments. While only one sensor streamer 106 is visiblein FIG. 3, it will be understood that many such sensor streamers 106 maybe present at similar depths but at different horizontal positions, andthus the further sensor streamers are not visible in FIG. 3. Inparticular, in FIG. 3 shows a sensor streamer 106 being towed in adirection indicated by arrow 300 by survey vessel 102, and sensorstreamer comprises a first section 302 and second section 304. Thesensor streamer 106 may comprise a plurality of sensors 116, a pluralityof streamer positioning devices 310 (which may have the samecharacteristics and functionality as streamer positioning devices 150and 152 of FIG. 1), and one or more acoustic positioning units 142.

In accordance with various embodiments, the localized depth of thesensor streamer may be controlled, at least in part, by respectivestreamer positioning devices 310. In one illustrative deep tow system asin FIG. 3 the depth of the first section 302 of the sensor streamer maychange linearly downward from a most shallow point to a most deep point.As illustrated, the depth of first section 302 may change essentiallylinearly (with expected fluctuations from currents and drag) from themost shallow point to the most deep point. In other embodiments, thedepth of first section 302 may likewise change parabolically,hyperbolically, or catenoidally. In still other embodiments, the mostshallow point of first section 302 may not be at the most proximalpoint, and/or the most deep point may not be at the most distal point.The depth of second section 304 may be constant and at least as deep asthe most deep point of section 302. The depth profile of the firstsection 302 may be controlled, at least in part, by supplying setpointdepths to the streamer positioning devices 310 within the second section302, where the setpoint depths may define the example profile. The depthprofile of the second section 304 may be controlled by supplyingsetpoint depths to the streamer positioning devices 310 associated withthe second section 304. Positioning devices 310 may, in at least someembodiments correspond to streamer positioning devices 150 and 152. Insome embodiments, section 302 may be 3,000 meters in length, and section304 may be 4,000 meters in length, but different sensor streamer lengthsmay be used in various embodiments. Moreover in the example system, theshallowest point may be about 20 meters, and the depth may changelinearly in this example to deepest tow depth of about 30 meters, butother depths may be used.

Still referring to FIG. 3, in one example deep tow system, the sensorstreamer 106 may couple at its distal end to a drag body 160. As thename somewhat implies, drag body 160 may provide a predetermined amountof drag at the distal end of the sensor streamer 106. The drag may helpkeep the sensor streamer in a proper orientation relative to the surveyvessel 102 and or other sensor streamers (not visible in FIG. 3). Asshown, however, the drag body 160 is not a tail buoy, and does not ridealong the surface of the water during the marine geophysical survey,thus eliminating the mechanical noise imparted to sensor streamer 106and the chance of entanglement with other survey equipment. Thediscussion now turns to an explanation of an example drag body 160 ingreater detail.

FIG. 4A shows a side elevation view of an embodiment of drag body 160 infurther detail. Drag body 160 may be configured to mechanically coupleto a distal end 402 of sensor streamer 106 using a connector 404. Insome embodiments, connector 404 may comprise a fixed portion 406 coupledto a rotatable portion 408. The fixed portion 406 and rotatable portion408 define a central axis 410 which may be coaxial with the central axisof sensor streamer 106 and the drag body 160. The fixed portion 406 maybe rigidly coupled to the drag body 160, and the rotatable portion 408may be coupled to the fixed portion in such a way that the connector 404can swivel, enabling drag body 160 to rotate freely relative to sensorstreamer 106. Connector 404 may include power, electrical and/or opticalconductors to supply power and/or telemetry to drag body 160 from surveyvessel 102 (either directly or indirectly via other survey equipment).

The drag body 160 may include an outer hull 412 that defines an exteriorsurface 414. In some embodiments, exterior surface 414 exhibits acircular diameter, a leading end 416, a length L, a diameter D, and atrailing end 418. In at least some cases, and as shown, the exteriorsurface defines a “torpedo” shape defining a rounded leading end 416 andtapered trailing end 418, but other shapes for the exterior surface 414(e.g., spherical, cubical, conical etc.) may be equivalently used. Inthe example system, outer hull 412 has a length L greater than thediameter D. In some cases, the length L may be at least twice thediameter D. In one example system, the diameter may be on the order ofabout one meter, and the length may be on the order of about threemeters, but again different diameters and lengths may be equivalentlyused. As would be understood by one of ordinary skill in the art withthe benefit of this disclosure, the shape and dimensions of exteriorsurface 414 may be selected to meet the expected operational dragrequirements. In some cases, the outer hull 412 may be constructed of ametallic material; however, depending on the operational depth of themarine geophysical survey, the outer hull 412 could be made of anysuitable material (e.g., plastic, fiberglass).

FIG. 4A further shows an optional tail fin system 420 coupled to thetrailing end 418 of the outer hull 412. The tail fin system 420 mayenhance directional stability of the drag body 160 when in use.

FIG. 4B shows a side elevation, cross-sectional view, of the drag body160 in accordance with at least some embodiments. In particular, theouter hull 412 defines an internal volume 430. Within the internalvolume 430 may reside an electronics enclosure 432. The electronicsenclosure 432 may define a sealed internal volume, and within which mayreside various electronic components. For example, the drag body 160 mayhave an inertial navigation system 434 (shown in block diagram form, anddiscussed more below) disposed within the electronics enclosure 432. Thedrag body may also have a telemetry unit 436 and a relative positioningunit 438 disposed within the electronics enclosure 432. Other devicesmay also be disposed within the electronics enclosure (such as powercontrols), but are not shown so as not to unduly complicate the figure.While the electronics enclosure is shown disposed medially within theinternal volume 430 of the outer hull 412, the electronics enclosure maybe located at any suitable position.

Still referring to FIG. 4B, the drag body 160 may further define one ormore internal chambers, such as chamber 440, designed to displace apredefined amount of water to make the drag body neutrally buoyant, orany other buoyancy selected for desired operational characteristics ofthe drag body 160 in relation to the sensor streamer (not shown in FIG.4B). While in some cases the buoyancy of the drag body 160 may beadjusted at the surface prior to deployment (e.g., by partially fillingchamber 440 with water), during normal use the operational buoyancy ofthe drag body 160 remains constant. Stated otherwise, in example systemsthe drag body 160 is not configured to rise or sink on command—thebuoyancy while the drag body 160 is operationally deployed does notchange by operation of devices within the drag body 160. It is notedthat buoyancy may change based on external factors (e.g., depth, watertemperature, salinity), but buoyancy changes based on external factorsshall not be considered a change of the constant buoyancy of the dragbody 160.

Finally with respect to FIG. 4B, the example drag body device 160 mayfurther comprise a retriever system 450 at least partially disposedwithin the internal volume 430. In abnormal situations during a marineseismic survey a sensor streamer may become disconnected from the othersurvey equipment. Disconnection may be a mechanical failure associatedwith a sensor (e.g., at a lead-in connector), or disconnection may becaused by separation of the sensor streamer (e.g., cut by propeller ofpassing ship, shark bite). Once a sensor streamer and associated dragbody 160 have become disconnected, the sensor streamer may becomenegatively buoyant and thus sink to the bottom, and possibly lost. Toavoid loss of the drag body 160 and remaining portion of the sensorstreamer, the retriever system 450 may monitor depth of the drag body160. When depth of the drag body 160 passes a predetermined depth belowany expected operational depth, the retriever system 450 may take actionto make the drag body 160 positively buoyant. The action taken to makethe drag body 160 positively buoyant may take many forms. For example,the retriever system may deploy a lifting bag system filled with gas toincrease buoyancy of the drag body 160. The retriever system 450 mayshed ballast weights coupled to the drag body 160 to increase buoyancy.As yet another example, the retriever system 450 may displace waterwithin the chamber 440 (e.g., increasing internal pressure to force thewater out of the chamber 440 through a check valve) to increase buoyancyof the drag body 160. The presence or use of the retriever system 450 asan emergency buoyancy system that activates to prevent loss the dragbody, however, shall not be considered to affect the constant“operational buoyancy” of the drag body 160 during normal use.

The specification now turns to operational methods of the drag body 160in accordance with example embodiments. In particular, the drag body 160associated with each sensor streamer may be initially held on the deckof a deployment vessel (e.g., survey vessel 102). While on deck, theinertial navigation system 434 may be provided a very precise positionindication, such as a position determined by a GPS device associatedwith and coupled to the vessel. The drag body 160 may then be deployedfrom the surface, and subsequently submerged to operational depth (e.g.,30 meters). In the example systems, the drag body 160 may stay submergedthe entire time the sensor streamer is operationally deployed. That is,the drag body 160 may not be raised to the surface again during theentire marine geophysical survey (in a deep tow, for example, bringingthe drag body to the surface is not practical). During periods of timewhen the drag body 160 is submerged, the inertial navigation system 434periodically (e.g., every second) may determine a calculated position ofthe drag body 160, and may relay the calculated position to the surveyvessel 102. However, inertial navigation systems experience positionalerrors or positional drift (sometimes referred to as integration drift),and the longer the inertial navigation system operates without receivinga highly accurate position indication from an external source, thegreater the positional drift in the calculated position may become.Because a drag body 160 in accordance with some embodiments remainsfully submerged while operationally deployed, a positioning system suchas GPS internal to the drag body 160 may not be included. Thus, reducingpositional drift of calculated position may be addressed in other ways,examples of which are discussed in greater detail below.

FIG. 5 shows, in block diagram form, various electrical and/orelectromechanical devices that may be (at least partially) disposedwithin the electronics enclosure 432 defined within the drag body 160.In particular, FIG. 5 shows the example drag body 160 may comprise aninertial navigation system 434. Inertial navigation system 434 maycomprise an inertial measurement unit 500 coupled to a computer system502. Each will be discussed in turn. The inertial measurement unit 500may comprise a six-axis accelerometer 504 and a three-axis gyroscope(gyro) 506. With the inertial measurement unit 500 disposed within thedrag body 160, six-axis accelerometer 504 detects the accelerations ofdrag body 160 along three translational axes and about three rotationalaxes as the drag body moves in the sea. Three-axis gyro 506 detects theinstantaneous rotational position of drag body 160 which may also beinduced by its motion in the sea. Output signals 508 and 510representing the values of the accelerations and rotational positions,respectively, may be input to a digitizer 512 to convert analog outputsto digital form which is provided to the computer system 502 to befurther processed. In an embodiment, the data, in digital form may becommunicated to computer system via an industry standard peripheral bus,such as a Universal Serial Bus (USB). Alternatively, any other suitableinterface may be used.

In the embodiment of FIG. 5, six-axis accelerometer 504 and three-axisgyro 506 are depicted as integral components of the inertial measurementunit 500. Inertial measurement unit 500 may be a commercially available“off-the-shelf” device, such as a Honeywell HG9900 inertial measurementunit from Honeywell Aerospace of Phoenix, Ariz. However, in other cases,six-axis accelerometer 504 and three-axis gyro 506 may be providedseparately. In such embodiments, the respective outputs may be digitizedseparately and separately communicated to processor 520, but theprinciples of the disclosure remain the same.

The computer system 502 may be coupled to the inertial measurement unit500 and may take any suitable form. In example cases, the computersystem comprise a processor 520 coupled to a memory device 522 (e.g.,random access memory, read only memory, flash memory, and combinationsthereof). Processor 520 may be implemented as an embedded centralprocessing unit a single board computer, an application specificintegrated circuit, a field programmable gate array, or the like. Thememory device 522 may store programs that are executed by the processor520 to perform the tasks associated with calculating position by theinertial navigation system 434.

Processor 520, executing programs stored on the memory device 522, mayintegrate the accelerations measured by accelerometer 504 to determine aposition of drag body 160. Note that the position may include componentsdue to the translational motion of the drag body through the sea andcomponents due to the pitch, yaw and roll of the drag body in the sea.The rotational position data from three-axis gyro 506 may then be usedby processor 520 to correct for the pitch, yaw and roll of drag body 160and provide the calculated position data of the drag body based on aninitial or starting position (again as discussed above). Processor 520may then provide the calculated position data to telemetry unit 436which may be connected to the streamer telemetry network (not shown inFIG. 5) for transmission to survey vessel 102.

Additionally, telemetry unit 436 may receive telemetry data transmittedto the drag body from the survey vessel. Data that might be sent to thedrag body may include: the GPS position of the survey vessel during themarine seismic survey; speed of the vessel; calculated position datafrom other devices, such as other drag bodies within the marine seismicsurvey.

In yet another embodiment, computer system 502 may be omitted. In suchan embodiment, the digitized data from digitizer 512 may be communicateddirectly to telemetry unit 436 for further communication on the streamertelemetry network to survey vessel 102. A computer system on board thesurvey vessel may then determine the position of the drag body from theraw data as described above. In such an embodiment, the electrical powerthat is provided to the drag body may be reduced. However, the amount ofdata that is communicated on the streamer telemetry network may beincreased.

As discussed above, during periods of time when the drag body 160 issubmerged, the inertial navigation system 434 periodically (e.g., everysecond) may determine a calculated position of the drag body 160, andmay relay the calculated position to the survey vessel 102. However,inertial navigation systems experience positional drift, and the longerthe inertial navigation system operates without a receiving a highlyaccurate position indication from an external source, the greater thepositional drift in the calculated position may become. Because a dragbody 160 in accordance with example systems remains fully submergedwhile operationally deployed, and because a GPS internal to the dragbody 160 may not be included, positional drift of calculated positionmay be addressed in other ways. The specification now turns to exampleembodiments of reducing positional drift.

The various electronics at least partially disposed within theelectronics enclosure may further include a relative positioning unit438 in the form of an acoustic/optic positioning unit, a compass 528,and a various sensors 530 (e.g., depth sensor, temperature sensor,salinity sensor). These devices may be used to provide additionalinformation with respect to the position and orientation of the dragbody. Such additional information may be used to adjust the calculatedposition determined using the data from the inertial measurement unit500, to at least in part, compensate for positional drift. Suchcompensation may be performed by processor 520 of the computer system502, which may be coupled to relative positioning unit 438 and compass528 to receive data indicative of position and orientation datatherefrom. Various sensors 530 may be coupled to relative positioningunit 438 and may provide information on the depth of the drag body andthe temperature and salinity of the sea water in the vicinity thereof tothe relative positioning unit.

As previously described, a typical survey vessel may tow a plurality ofsensor streamers. For example, in at least some embodiments from one upto about twenty-four sensor streamers may be towed. At least one of thesensor streamers may be equipped with a drag body 160 including aninertial navigating system in accordance with the principles disclosedherein. Each drag body may be attached to the distal end of itsrespective sensor streamer 106. Upon deployment of the sensor streamer106, the operational length of sensor streamer 106 will be known to anoperational tolerance. Assume, for the purpose of illustration, that thesurvey vessel is underway in a current-free water body. Then, the sensorstreamers 106 will be arrayed parallel to the direction of traveldirectly behind the vessel, and for any given arrangement of thestreamers transverse to the direction of travel, the spacing of thestreamers, and thus the spacing of the drag bodies 160 is known.Consequently, any deviation of the distance between pairs of drag bodiesfrom that based on the streamer spacing may be attributed to positionaldrifts in the inertial navigation systems. Thus, consider by way ofexample, and for ease of illustration, a tow along in a northerlydirection. Because the sensor streamers are of fixed length and the dragbodies 160 are at the end of the streamer, a deviation in the calculatedposition of the drag body along a north-south line that corresponds toan apparent change in distance between the end of the streamer proximalto the vessel and the drag body may be attributed to positional drift ofthe inertial navigation system, and may be used to provide an estimateof the rate of drift of the guidance system and the reported positionsof the respective drag bodies may be corrected thereby. Although theforegoing has been described in conjunction with a survey vesseltraveling a linear course, similar considerations would be expected ifthe vessel were traveling on a circular or similar curved course.

If ocean currents are present, the sensor streamers may be deflected bythe current and additional positioning mechanism may be used to estimateand correct for positional drift. To that end, acoustic positioning unit438 may be configured to transmit and receive acoustic signals throughthe water. Further, streamers 106 may also be equipped with acousticpositioning units 142 distributed at locations along the length thereof,as described above. In particular, in at least some embodiments, theacoustic positioning units in streamers 106 may be located atpredetermined intervals along the respective streamer. In someembodiments, acoustic positioning unit 438 may interrogate the acousticpositioning units in streamers 106. In at least some embodiments, a dragbody 160 may interrogate a plurality of acoustic positioning units in astreamer 106, and, further, in at least some embodiments may interrogatea plurality of acoustic positioning units in a plurality of streamers106. Each acoustic positioning unit may be interrogated, in turn via acommunication signal addressed to the positioning unit beinginterrogated. The unit being interrogated may then return a response tothe interrogating drag body 160.

Further, in at least some embodiments, an acoustic positioning unit 438in a particular drag body 160 may interrogate the acoustic positioningunits in the other drag bodies attached to respective ones of theplurality of streamers 106. Each such drag body 160 may be interrogated,in turn via a communication signal addressed, or otherwise directed, tothe drag body being interrogated. The interrogated drag body may thentransmit a response which may be received by the interrogating drag body160. Using the elapsed time between the launching of the interrogationsignal into the water and the receipt of the response, the acousticpositioning unit 438 may determine the distance between theinterrogating drag body and the interrogated acoustic positioning unitdeployed in a streamer 106 or drag body 160. The distance may bedetermined using the measured elapsed time and the velocity of sound inthe surrounding sea water. In this respect, acoustic/optic positioningunit 518 may use the data received from sensors 530 to account for theeffect of pressure, temperature and salt content on the velocity ofsound in seawater. Further, while in the illustrated embodiment in FIG.5 depicts the sensors 530 coupled to positioning unit 438, sensors 530may, in at least some embodiments, be coupled to processor 520 and thecompensation for the effects of pressure, temperature and salt contentmade by processor 520 on the position data received from positioningunit 438.

Regardless of whether the sensors 530 couple directly to the positioningunit 438 or to the processor 520, positioning unit 438 may provide dataindicative of distance of a particular drag body with respect to each ofthe other drag bodies. Thus, each drag body may have an acousticallydetermined distance with respect to a plurality of positions along oneor more streamers and with respect to one or more other drag bodies.These acoustically determined distances may be indicative of therelative positions of the drag body and the locations on the streamers,and the other drag bodies, as the case may be.

Each of the acoustically determined distances defines a circle with itscenter at the respective location of the corresponding acousticpositioning unit on a sensor streamer or relative positioning unit 438on a drag body. The position of a particular drag body may be determinedfrom such circles about the acoustic positioning units 142 or relativepositioning unit 438 interrogated by that body. In particular, theposition of the drag body must lie on each of such circles, and thus theposition may be found from the intersections, or crossings, thereof.Because a pair of circles may intersect in two points, there may beambiguities in the position of the particular drag body. By using thedistances from a plurality of acoustic or relative positioning units anda plurality of sensor streamers, ambiguities may be resolved. Althoughthe foregoing embodiment is described in terms of an acousticpositioning determination, other distance determination mechanisms maybe used. For example, rather than acoustic sound an opticalinterrogation and response link may be employed, which may be used in atleast some embodiments in accordance with the principles described inreference to the acoustic positioning unit.

The relative positions of the drag body may be used to correct for thepositional drift of the inertial navigation system 434. For example, thepositions of the drag body relative to the acoustic positioning unitlocations on the sensor streamers or relative positioning unit locationson other drag bodies may not be expected to have a secular variation(i.e., one that is substantially monotonic increasing or decreasing).Thus, a secular variation of the relative position as determined by theinertial navigation system 434 may be attributed to positional drift ofthe inertial navigation system 434. The position of the drag body asdetermined by the inertial navigation system 434 may thus be correctedby adjusting for the positional drift. Additionally, by incorporatingthe relative positions from multiple locations and other drag bodies,errors in estimating the drift may be reduced by, for example averagingthe individually estimated drifts.

Still referring to FIG. 5, additional information may be obtained fromcompass 528. As the drag body is towed through the water at the distalend of its sensor streamer, heading may deviate from the survey vesselheading because of currents deflecting the sensor streamer, and, furtherthe heading may vary in time from yaw of the drag body at the connector404. Compass 528 may provide heading data (corrected for declination)for the drag body which may be compared to the heading as determinedfrom the data output from inertial measurement unit 500. Anotherestimate of the positional drift of inertial navigation system 434 maythereby be obtained, and in an embodiment, combined with the estimatesdetermined from the relative position data as described above.Alternatively, in still other embodiments, the estimate of thepositional drift obtained from the heading information may beindependently used to correct for the positional drift of the inertialnavigation system 434.

In yet still further embodiments, the speed and position of the surveyvessel may be transmitted to the drag body via telemetry unit 436, forexample, through a conductor 550 defined within the connector 404.Comparing the average speed of the drag body with the speed of thesurvey vessel, as may be received via telemetry unit 436, providesadditional information with respect to the positional drift of theinertial navigation system 434. Further still, comparing the vesselposition with the corrected drag body position provides information onthe deflection of the sensor streamers from subsurface currents at thetowing depth, which may be used in updating position corrections overthe duration of the survey.

In yet still further embodiments, a drag body may communicate itscalculated position to the other drag bodies via telemetry unit 436, forexample. Telemetry unit 436 may send the current position of the dragbody to survey vessel 102 which may then relay the information to theother drag bodies, or the streamer network may enable directcommunication between the drag bodies. Each receiving drag body may thenuse its calculated position as determined by its inertial navigationsystem and the relative position data determined as described above todetermine an estimated position of the other drag bodies. The receivingdrag body may thereby realize a consistency check by comparing with therelayed positions of the other drag bodies, and the respective estimatedpositions.

FIG. 6 shows, in block diagram form, additional features of a drag bodywith an inertial navigation system in accordance with exampleembodiments. In particular, FIG. 6 shows inertial navigation components600 (which may include the inertial navigation system 434, relativepositioning unit 438, telemetry unit 436, various sensors 530, andcompass 528). The inertial navigation components 600 couple to a powercontrol unit 602 which provides power to inertial navigation components600. Power control unit 602 may receive electrical power from severalsources, including a battery 606, hydroelectric generator 610, and aconnection 604 to the streamer power bus. Power control unit 602 mayswitch between the sources. Further, in the various embodiments, not allof these sources may be provided or otherwise available, and the powercontrol unit may selectively switch between power sources based ondetecting their presence or availability. Thus for example, if streamerpower is not provided, or fails, power control unit 602 may switchbetween battery 606 and hydroelectric generator 610, which may have acontrol line 612, discussed further below, coupled to power control unit602 along with power connection 614. The selection of either battery 606or hydroelectric generator 610 may be made in accordance with apredetermined priority. In at least some embodiments, battery power maytake priority over hydroelectric generation, for example, to avoid theadditional drag that will accompany the generation of electrical powerby generator 610 which proceeds by the turning of the generator shaft bywater flowing through a propeller attached to the shaft. Additionally,power control unit 602 may monitor the state of the battery 606, andsupply electrical power to battery charger 608 to recharge the battery.If streamer power is available, power control 602 may provide streamerpower to battery charger 608. Otherwise, power control 602 may providepower to charger 608 via hydroelectric generator 610. In at least someembodiments, when hydroelectric generator 610 is not in use, powercontrol unit 602 may send a control signal, via control line 612, to thegenerator to feather the propeller to further reduce drag.

Refer now to FIGS. 7A and 7B illustrating a flow chart of a method 700to of determining a position of a towed drag body in accordance with theprinciples described above. In FIG. 7A, method 700 starts in block 702.In block 704, the drag body is towed behind a survey vessel. Acalculated position of the drag body is determined in block 706, forexample from the data output from inertial measurement unit 500. Inblock 708, the position is corrected, using a relative positioning unitin the drag body providing a relative position of the drag body.Inasmuch as, in various embodiments, the correction of position takesplace without using a GPS associated with the drag body, the correctiontakes place while the drag body is fully submerged.

FIG. 7B illustrates block 708 in further detail. Data indicative of theposition of the drag body relative to a location on a sensor streamer isreceived, block 7082. In block 7084, data indicative of the position ofthe drag body relative to another drag body is received. In at leastsome embodiments, the data may be received in blocks 7082 or 7084 inresponse to an interrogation by the relative position unit in the dragbody as described in conjunction with FIG. 5. In blocks 7086 and 7088,respectively, data indicative of the speed and position of the surveyvessel are received. The aforementioned data may, in at least someembodiments, be received via telemetry unit 436. Data indicative of theheading of the drag body is received in block 7090, and block 708proceeds to block 710.

As described hereinabove, in the exemplary context of determining therelative position of drag bodies wherein the relative positions may beused to correct for drift of an inertial navigation system. As set forththerein, optical positioning techniques may be used in that context todetermine the relative positions of the drag bodies. More generally,optical positioning techniques may be used to determine the relativepositions of other in-sea equipment, such as seismic sources,electromagnetic sources, source sub-arrays, paravanes, etc. Suchtechniques will be described in conjunction with FIGS. 8-10, below.

Refer now to FIG. 8 showing a block diagram of optical system 800 inaccordance with at least some embodiments. Optical system 800 includesan optical relative positioning unit 802 coupled to an underwaterimaging device 804. In at least some embodiments, underwater imagingdevice 804 may comprise a digital still camera that takes a sequence orstream of images configured for underwater operation and in otherembodiments a video camera configured for underwater operation. Computersystem 502 may be coupled to the optical relative positioning unit 802and may take any suitable form. In example cases, the computer systemcomprise a processor 520 coupled to a memory device 522 (e.g., randomaccess memory, read only memory, flash memory, and combinationsthereof). Processor 520 may be implemented as an embedded centralprocessing unit, a single board computer, an application specificintegrated circuit, a field programmable gate array, or the like. Thememory device 522 may store programs that are executed by the processor520 to perform the tasks associated with calculating a relative positionusing data from optical relative positioning unit 802, as describedfurther below in conjunction with FIG. 10. Although processor 520 isshown in FIG. 8 as a separate unit from optical relative positioningunit 802, in at least some embodiments, processor 520 may be integratedwith optical relative positioning unit. Likewise, memory device 522 may,in still other embodiments, also be integrated with optical relativepositioning unit 802.

In yet another embodiment, computer system 502 may be omitted. In suchan embodiment, the data from optical relative positioning unit 802,which may be in digital form, may be communicated directly to telemetryunit 436 for further communication to the survey vessel 102. However,the amount of data that is communicated on the streamer telemetrynetwork may be increased.

Additionally, telemetry unit 436 may receive telemetry data transmittedto the drag body (if present) from the survey vessel. Data that might besent to the drag body may include: the GPS position of the survey vesselduring the marine geophysical survey; speed of the vessel; calculatedposition data from other devices, such as other drag bodies within themarine geophysical survey.

Underwater imaging device 804 may be deployed to view the underseaenvironment in the vicinity of a drag body, seismic source,electromagnetic sources, source sub-arrays, paravanes, or other in-seaequipment. For example, if underwater imaging device 804 is deployed inconjunction with a drag body, underwater imaging device 804 may bedeployed externally, attached to outer hull 412 of the drag body asdepicted in FIG. 9A showing a side elevation view of a drag body 160 inaccordance with at least some embodiments. Further, drag body 160 mayhave a target pattern 902 affixed to outer hull 412. For example, targetpattern 902 may be painted on outer hull 412. A painted target patternmay be comprised of a reflective paint to facilitate acquisition of thetarget pattern (as described further below) in reduced light conditions.In other embodiments, target pattern 902 may be comprised of activelighting to facilitate distance determinations in low-ambient lightconditions. For example, target pattern 902 may be comprised of aplurality of light-emitting diodes (LED) disposed on the target objectso as to form the target pattern. Target pattern 902 may be affixed byany means suitable to a deployment of the drag body in a marineenvironment. The form of target pattern 902 is illustrative only, andmay take any configuration suitable for use in conjunction with themethodology described in conjunction with FIG. 10. In particular, targetpattern 902 may include a plurality of fiducial portions having a knowndistance between them, which fiducial portions may be amenable toidentification by pattern recognition techniques. The illustrativetarget pattern 902 in FIG. 9A, may comprise a “hash” symbol (14) inwhich the vertical line portions and horizontal line portions,respectively, may provide fiducial portions, as described below.Further, target patterns affixed to different target objects mayincorporate unique features that may be used to particularly identifythe respective target object. For example, a machine readable numeralmay be incorporated into the target pattern.

Referring now to FIG. 9B, showing a cross section view through a sideelevation of a drag body 160 in accordance with some embodiments. InFIG. 9B, underwater imaging device 804 is deployed within internalvolume 430 of drag body 160. Underwater imaging device 804 may view theundersea environment through a transparent view port 904 of outer hull412. Internal volume 430 may comprise a camera dome 906, which may bedefined by view port 904 and partition 908 which may isolate underwaterimaging device 804 from chamber 440. In a deployment within an internalvolume of drag body, underwater imaging device 804 may be protected fromthe underwater environment and, in at least some embodiments, comprise adigital still camera that takes a sequence or stream of images and in atleast some other embodiments a video camera.

In either an external deployment of underwater imaging device 804 as inFIG. 9A or an internal deployment as in FIG. 9B, underwater imagingdevice 804 may be provided with a remotely controllable panning mount tofacilitate acquisition of a target object from which the relativedistance between the target object and the drag body may be determined.The panning of underwater imaging device 804 may be under the control ofoptical relative positioning unit 802 as described further below inconjunction with FIG. 10.

Turning now to FIG. 10, FIG. 10 shows a flowchart of a method 1000 fordetermining a distance and/or bearing between in-sea equipment such asseismic sources, source units, electromagnetic sources, sourcesub-arrays, paravanes, and/or drag bodies (simply referred to as a“target object”). Method 1000 may be performed by optical relativepositioning unit 802. Alternatively, in at least some embodiments,method 1000 may be performed by optical relative positioning unit 802 inconjunction with computer system 502. Method 1000 starts at block 1002and in block 1004 captures an image stream from underwater imagingdevice, such as an underwater video camera or in at least some otherembodiments an underwater digital still camera that takes a sequence orstream of images. In block 1006, a target acquisition search isperformed. Target acquisition analysis may be performed using patternrecognition techniques, such as a neural network trained on apredetermined target pattern affixed to the target object. However, thesophistication of a neural network may not be necessary and any suitablepattern recognition technique may be used. In this respect, having aknown target pattern, which may be, for example, depicted in highcontrast with respect to the background, may be exploited by the patternrecognition technique. For example, an edge detection algorithm may beused to recognize the target pattern in the image stream. One suchalgorithm that may be used in at least some embodiments is the Cannyedge detector.

If a target is acquired, block 1008, method 1000 proceeds by the “Yes”branch of block 1008. Otherwise, method 1000 proceeds by the “No” branchof block 1008 to return to block 1006, via block 1010, to acquire thetarget. In block 1010 the underwater imaging device may be panned tofacilitate target acquisition. Panning of the underwater imaging devicemay be, in at least some embodiments, under the control of opticalrelative positioning unit 802.

Returning to the “Yes” branch of block 1008, upon acquisition of thetarget, method 1000 tracks the target in block 1012. In block 1014, animage of the target from the image stream is captured, using, forexample a frame grabber. In at least some embodiments a frame grabbermay be incorporated in software stored in memory device 522. In at leastsome alternative embodiments, a frame grabber may comprise hardwareincluded in optical relative positioning unit 802.

In block 1016, the apparent size of the target pattern is measured. Theapparent size of the target pattern may be determined by, for example,counting the number of pixels spanned by features in the target pattern,such as the number of pixels between fiducial portions identified in thetarget pattern.

In block 1018, the distance from the target object to the underwaterimaging device is calculated using the apparent size of the targetpattern measured in block 1016, the known actual size of the targetpattern and a calibration of the underwater imaging device. For example,a calibration may comprise a count of the number of pixels betweenfiducials in a target pattern at a preselected distance from theunderwater imaging device; the number of pixels comprising a calibratedimage size of the target pattern. The calibration may further comprisethe preselected distance. In at least some embodiments, the calibrationmay be stored in memory device 522. A distance between a target objectand the underwater imaging device may then be obtained by a linearscaling: multiplying the ratio of the number of pixels in thecalibration to the number of pixels in the captured image of the targetpattern on the target object by the preselected distance in calibration.Thus, by way of illustration only, if the calibrated image size inpixels (px) in the target pattern in the calibration is say 500 px andthe number of pixels spanning the feature in the captured image is, forexample, 250 px, and with a calibration distance of 15 m, the distancebetween the target object and the underwater imaging device would be 30m. The distance to the underwater imaging device is indicative ofdistance between the target object and the in-sea equipment includingthe underwater imaging device.

Further, in at least some embodiments, a bearing to the target objectfrom the underwater imaging device may also be determined in block 1018.For example, in an embodiment of method 1000 used in conjunction withsource units, a bearing and distance to a particular seismic source inthe source sub-array may be determined. Such source sub-arraypositioning information, that is, distance and bearing, may be used in ageophysical survey in a quality control aspect for example. As such, thepositioning information may be used to evaluate whether the pressuredifference between the pressure signature output of a particularsub-array and the output of a nominal source sub-array is within aspecified range. In another aspect, positioning information may be usedin designing a de-signature filter that is used in processing theseismic data. The de-signature filter is applied to eliminate the sourcesignature footprint from the seismic data. In related art systems,underwater acoustics may be used to obtaining the source sub-arraypositioning information. However, the underwater acoustics may beunreliable as it is sensitive to air bubbles, for example. Opticaldetermination of positioning information may mitigate reliability issuesassociated with underwater acoustic determinations of the positioninginformation.

As described below in conjunction with FIG. 11, a plurality of seismicsources may be deployed in a linear sub-array. In determining a set ofdistances to each of the seismic sources in the plurality, the line ofsight between the underwater imaging devices may vary, beingperpendicular to the line formed by the linear sub-array with respect toone of the plurality of sonic sources, the line of sight becoming skewedwith respect to the line formed by the linear sub-array as the line ofsight moves to the right or left from the perpendicular direction. Thegeometry of the distance and bearing determination will be described inconjunction with FIG. 10A below.

For the purpose of illustrating the principles herein, a horizontalbearing to the underwater imaging device from the target object may beconsidered. The exemplary target pattern comprising a hash symbol asdescribed above in conjunction with FIG. 9A may be used. The bearing maybe determined by application of the law of cosines to a triangle formedby the known distance between the vertical portions of the hash symboland the distances to each vertical portion determined as previouslydescribed.

Refer to FIG. 10A which schematically illustrates aforesaid applicationof the law of cosines to a determination of the horizontal bearing.Points 1050 and 1052 represent the relative positions of verticalportions of the target pattern, having a distance b therebetween, onrespective lines of sight 1054 and 1056 from underwater imaging device804. The distance b may be determined from an actual size of the targetpattern. The respective angles between the horizontal 1058 to lines ofsight 1054 and 1056 are denoted β and γ, respectively. The distancebetween underwater imaging device 804 and the target pattern verticalportion along line of sight 1052 is denoted by a, and along line ofsight 1054 by c. Then applying the law of cosines to horizontal 1058 andline of sight 1054 and angle β therebetween yields Equation 1 for theangle β:

$\begin{matrix}{\beta = {{\arccos\left\lbrack \frac{a^{2} + b^{2} - c^{2}}{2\;{ab}} \right\rbrack}.}} & {{Eq}.\mspace{14mu} 1}\end{matrix}$Likewise, applying the law of cosines applied to horizontal 1058, lineof sight 1056 and the angle γ therebetween yields Equation 2 for theangle γ:

$\begin{matrix}{\gamma = {{arc}\;{{\cos\left\lbrack \frac{b^{2} + c^{2} - a^{2}}{2\;{ac}} \right\rbrack}.}}} & {{Eq}.\mspace{14mu} 2}\end{matrix}$The bearing to the target object may then be determined from theaverage, Equation 3:horizontal bearing=0.5(β+(180−γ)).  Eq.3In Equations 1 and 2, arccos denotes the inverse cosine function. Avertical bearing may likewise be determined using the known distancebetween horizontal fiducial portions of the target pattern, such as thehorizontal lines in a target pattern comprising a hash symbol.

Returning to FIG. 10, in block 1020, the distance and bearing determinedin block 1018 are output. In at least some embodiment, the distance andbearing may be output to telemetry unit 436 for transmission to thesurvey vessel, for example. In other embodiments, distance and bearingmay be output to computer system 502 and seismic source separationdetermined onboard the in-sea equipment platform including opticalsystem 800. In still other embodiments, the distance may be output tocomputer system 502 and used to correct the position of in-sea equipmentfor drift of an inertial navigation system within the in-sea equipmentas described hereinabove in conjunction with FIG. 7.

In block 1022, method 1000 may be selectably terminated. For example, ifno further distance determinations are needed, the survey vessel maysend a signal, via telemetry unit 436, say, to optical system 800 toterminate distance measurements. Method 1000, in block 1022 determinesif distance measurements are to terminate. If so, block 1022 fallsthrough the “Yes” branch to block 1024. In block 1024, capture of theimage stream is terminated, and method 1000 ends at block 1026. If, inblock 1022 it is determined that further distance measurements are to bemade, block 1022 proceeds by the “No” branch to block 1028. If a newtarget is to be acquired and a distance determination made with respectthereto, block 1028 proceeds by the “Yes” branch to block 1006. A newtarget may be selectably acquired via block 1028 by, for example asignal from the survey vessel. Alternatively, new targets may beacquired and distance determinations made with respect thereto inaccordance with a preprogrammed sequence of targets. If a new target isnot to be acquired, block 1028 proceeds by the “No” branch to block1012, and distance determinations continue with respect to the currenttarget.

Turn now to FIG. 11 illustrating a source unit 131 in accordance with atleast some embodiments. Source unit 131 may include a flotation device1102 and a beam 1104 disposed beneath and attached to the flotationdevice. Flotation device 1102 may, when source unit 131 is deployed,float on or near the sea surface. Alternatively, in at least someembodiments, flotation device 1102 may be ballasted during deployment sothat source unit 131 is substantially neutrally buoyant, whereby sourceunit 131 may be towed at a preselected depth beneath the sea surface. Asubframe 1106 may be suspended by a plurality of cables 1108 from beam1104. Source unit 131 may include a one or more seismic sources, e.g.air guns, 1110. The seismic sources may be suspended from subframe 1106by chains 1112. In the illustrative embodiment of FIG. 11, seismicsources 1110 may be distributed in a linear sub-array along subframe1106.

Further, source unit 131 may include target panels 1114 having a targetpattern disposed thereon. Target panels may be used if the size of thetarget pattern may be otherwise constrained if the target pattern wereaffixed directly to the subframe 1106, for example. Additionallyemploying a target panel 1114 may provide for interchangeability of atarget pattern and/or variation in contrasting background and targetpattern colors. In the illustrative embodiment of source unit 131 inFIG. 11, the target pattern is represented by the “hash” symbol. Aspreviously described in conjunction with FIGS. 9A, B and 10, 10A, thetarget pattern may be used to determine the distance and the bearingbetween source unit 131 and an underwater imaging device, such asunderwater imaging device 804, which may be coupled to the survey vesselthrough umbilical cable 133. In this way underwater imaging device 804may transmit an image stream comprising frames including images of atarget pattern on other in-sea equipment, for example, seismic sourceson another source unit or a drag body. The image stream may be processedby an optical relative positioning unit that is disposed remotely fromsource unit 131 and underwater imaging device 804. For example, theimage stream may be may be processed by an optical relative positioningunit and/or computer system on board the survey vessel.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Various advantages of the present disclosurehave been described herein, but embodiments may provide some, all, ornone of such advantages, or may provide other advantages.

It is noted that while theoretically possible to perform some or all thecalculations and analysis by a human using only pencil and paper, thetime measurements for human-based performance of such tasks may rangefrom man-days to man-years, if not more. Thus, this paragraph shallserve as support for any claim limitation now existing, or later added,setting forth that the period of time to perform any task describedherein less than the time required to perform the task by hand, lessthan half the time to perform the task by hand, and less than onequarter of the time to perform the task by hand, where “by hand” shallrefer to performing the work using exclusively pencil and paper.

From the description provided herein, those skilled in the art arereadily able to combine software created as described with appropriategeneral-purpose or special-purpose computer hardware to create acomputer system and/or computer sub-components in accordance with thevarious embodiments, to create a computer system and/or computersub-components for carrying out the methods of the various embodimentsand/or to create a computer-readable media that stores a softwareprogram to implement the method aspects of the various embodiments.

References to “one embodiment”, “an embodiment”, “a particularembodiment”, “example embodiments”, “some embodiments”, and the like,indicate that a particular element or characteristic is included in atleast one embodiment of the invention. Although the phrases “in oneembodiment”, “an embodiment”, “a particular embodiment”, “exampleembodiments, “some embodiments”, and the like, may appear in variousplaces, these do not necessarily refer to the same embodiment.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

What is claimed is:
 1. A method comprising: obtaining, at an underwaterimaging device, a stream of images of geophysical surveying equipment,wherein the geophysical surveying equipment includes a target pattern,the target pattern having a calibrated image size; tracking thegeophysical surveying equipment using the image stream and the targetpattern; capturing from the image stream, an image from the stream ofimages of the geophysical surveying equipment, wherein the imagecomprises an image of the target pattern having an apparent size; anddetermining, using a single image captured from the image stream, adistance between the underwater imaging device and the geophysicalsurveying equipment, the determining based on the calibrated image sizeand the apparent size of the target pattern in the single image.
 2. Themethod of claim 1 wherein the determining based on the calibrated imagesize and the apparent size comprises measuring the apparent size of thetarget pattern.
 3. The method of claim 2 wherein measuring the apparentsize of the target pattern comprises determining a number of pixelsbetween a first fiducial portion and a second fiducial portion in theimage of the target pattern, the number of pixels forming a measurementof the apparent size of the target pattern.
 4. The method of claim 3wherein the determining based on the calibrated image size and theapparent size further comprises: dividing a number of pixels between thefirst fiducial portion and the second fiducial portion in the calibratedimage size by the measurement of the apparent size of the target patternto form a ratio; and multiplying a calibration distance corresponding tothe calibrated image size by the ratio.
 5. The method of claim 1 furthercomprising outputting the distance between the underwater imaging deviceand the geophysical surveying equipment to a telemetry unit.
 6. Themethod of claim 1 wherein the underwater imaging device comprises anunderwater imaging device selected from the group consisting of adigital still camera and a video camera.
 7. The method of claim 1wherein: the underwater imaging device is a component of a drag bodycomprising an onboard inertial navigation system; and the method furthercomprises correcting, using the distance, a position of the drag bodydetermined using the onboard inertial navigation system.
 8. The methodof claim 1 wherein the geophysical surveying equipment comprises a firstsource and a second source, wherein the underwater imaging device isdisposed on the first source and the target pattern is disposed on thesecond source, the distance between the underwater imaging device andthe geophysical surveying equipment comprising a distance between thefirst and second sources.
 9. The method of claim 8, wherein each of thefirst source and the second source comprise a seismic source in a singlesource unit.
 10. The method of claim 8, wherein the first source and thesecond source comprise seismic sources in two different source units.11. The method of claim 8, wherein at least one of the first source andthe second source comprises an electromagnetic source.
 12. The method ofclaim 1 further comprising determining a bearing between the underwaterimaging device and the geophysical surveying equipment using the actualsize of the target pattern and the distance between the underwaterimaging device and the geophysical surveying equipment.
 13. A systemcomprising: a first geophysical survey equipment, the first geophysicalsurvey equipment including a target pattern, the target pattern having acalibrated image size; a second geophysical survey equipment, the secondgeophysical survey equipment including an underwater imaging device andan optical relative positioning unit coupled to the underwater imagingdevice, and wherein the optical relative positioning unit is configuredto: capture an image stream from the underwater imaging device; anddetermine a distance between the first geophysical survey equipment andthe second geophysical survey equipment using the calibrated image sizeof the target pattern and an apparent size of the target pattern using asingle image of the target pattern in the image stream.
 14. The systemof claim 13 wherein the optical relative positioning unit is furtherconfigured acquire the target pattern in the image stream.
 15. Thesystem of claim 13 further comprising a computer system coupled to theoptical relative positioning unit and wherein optical relativepositioning unit is configured to determine the distance between thefirst geophysical survey equipment and the second geophysical surveyequipment using the computer system.
 16. The system of claim 15 whereinthe computer system is configured to measure the apparent size of thetarget pattern in the single image of the target pattern and determinethe distance between the first and second geophysical survey equipmentusing the measured apparent size of the target pattern and thecalibrated image size of the target pattern at a calibration distancebetween the target pattern and the underwater imaging device.
 17. Thesystem of claim 13 wherein the first geophysical survey equipment andthe second geophysical survey equipment comprise a first and second dragbody, respectively, the first drag body having an outer hull, the outerhull having the target pattern affixed thereto, and wherein the seconddrag body comprises: an outer hull defining an internal volume, theouter hull including a transparent view port; wherein the underwaterimaging device is disposed within the internal volume and configured toview the target pattern through the transparent view port, and whereinthe distance between the first geophysical survey equipment and thesecond geophysical survey equipment comprises a distance between thefirst and second drag bodies.
 18. The system of claim 17 wherein thesecond drag body further comprises: an inertial navigation systemconfigured to output a linear acceleration and an angular acceleration;and a computer system coupled to the inertial navigation system, thecomputer system receiving the linear acceleration and the angularacceleration from the inertial navigation system and determining aposition of the second drag body in response to the linear accelerationand the angular acceleration, the position of the second drag body, theposition of the second drag body including a drift of the inertialnavigation system, and wherein the computer system is configured tocorrect the drift of the position of the second drag body using thedistance between the first and second drag bodies.
 19. The system ofclaim 13, wherein each of the first geophysical survey equipment and thesecond geophysical survey equipment comprise a seismic source in asingle source unit.
 20. The system of claim 13, wherein the firstgeophysical survey equipment and the second geophysical survey equipmentcomprise seismic sources in two different source units.
 21. The systemof claim 13, wherein at least one of the first geophysical surveyequipment and the second geophysical survey equipment comprises anelectromagnetic source.
 22. The system of claim 20 wherein the targetpattern is disposed on an interchangeable target panel attached to atleast one of the seismic source units.
 23. A system comprising: a firstgeophysical survey equipment, the first geophysical survey equipmentincluding a target pattern, the target pattern having a calibrated imagesize and an actual size; a second geophysical survey equipment, thesecond geophysical survey equipment including an underwater imagingdevice; and optical relative positioning unit disposed remotely from thesecond geophysical survey equipment and communicatively coupled to theunderwater imaging device, and wherein the optical relative positioningunit is configured to: capture an image stream from the underwaterimaging device; and determine a distance between the first geophysicalsurvey equipment and the second geophysical survey equipment using thecalibrated image size of the target pattern and an apparent size of thetarget pattern in a single image of the target pattern in the imagestream.
 24. The system of claim 23 wherein the first geophysical surveyequipment comprises a first source and the second geophysical surveyequipment comprises a second source.
 25. The system of claim 24, whereineach of the first source and the second source comprise a seismic sourcein a single source unit.
 26. The system of claim 24, wherein the firstsource and the second source comprise seismic sources in two differentsource units.
 27. The system of claim 24, wherein at least one of thefirst source and the second source comprises an electromagnetic source.28. The system of claim 23 wherein the optical relative positioning unitis disposed within a survey vessel communicatively coupled to the secondgeophysical survey equipment.
 29. The system of claim 23 wherein opticalpositioning unit determines the apparent size of the target pattern by:detecting the target pattern in the single image of the target pattern;detecting a fiducial portion in the detected target pattern; anddetermining a number of pixels spanned by the fiducial portion in thedetected target pattern.
 30. The system of claim 23 wherein the opticalrelative positioning unit is further configured to determine a bearingbetween first geophysical survey equipment and the second geophysicalsurvey equipment using the distance between the first geophysical surveyequipment and the second geophysical survey equipment and the actualsize of the target pattern.